The present invention relates to a flexible tubular pipe of the type used for the production and transport of fluids in the offshore oil industry. More precisely, it relates to pipes designed to transport gas or multiphase fluids.
Pipes intended for transporting a fluid in the offshore oil industry have been described in many patents of the Assignee hereof, such as, for example, EP 0 937 932. They comply with, among others, API (American Petroleum Institute) Recommended Practice 17B. The pipes are formed by a combination of various layers, each intended to allow the flexible pipe to withstand the service and handling stresses, and also the stresses specifically related to their offshore use. In particular, these layers comprise polymeric sheaths and reinforcement layers formed by the winding of profiled wires, strips or composite wires, but they may also include the winding of various tapes between the various reinforcement layers. More particularly, they include at least one internal sealing sheath or pressure sheath designed to carry the transported fluid. The sealing sheath may be the innermost element of the pipe and that pipe is called a smooth-bore pipe or the sheath may be placed around a carcass formed, for example from the short pitch winding of an interlocked strip and that pipe is then called a rough-bore pipe. Reinforcement layers formed from the winding of metal or composite wires are generally placed around the pressure sheath and may comprise, for example:                a pressure armor formed by the short-pitch winding of an interlocking profiled metal wire, said pressure armor being placed directly around the sealing sheath so as to take up the radial component of the internal pressure;        optionally, a hoop formed by the short-pitch winding of a non-interlocking profiled wire placed on top of the pressure armor in order to contribute to the internal pressure resistance, the pressure armor, with or without said hoop, forming what is called the pressure vault of the pipe; and        tensile armor plies formed by long-pitch windings of metal or composite profiled wires, said plies being intended to take up the axial component of the internal pressure and also the longitudinal stresses to which the pipe is subjected, such as for example the forces when laying said pipe.        
An external polymeric sheath or protective sheath is generally provided on top of the abovementioned reinforcement layers. In certain cases, an intermediate polymeric sheath is also provided. This intermediate sheath may, for example, be an “anti-collapse” sheath generally placed around the pressure vault. The purpose of this intermediate sheath is in particular to prevent the sealing sheath and the optional carcass that it surrounds from collapsing when the annulus (the space lying between the sealing sheath and the external sheath) is exposed to excessive pressure compared with the internal pressure of the fluid that it transports. This may be the case when the external sheath is damaged and no longer fluidtight, and therefore the hydrostatic pressure prevails in the annulus. This anti-collapse intermediate sheath is generally present in the case of a smooth-bore pipe since the sealing sheath is all the more liable to collapse when it is not supported by a carcass.
In flexible production pipes, the transported fluid is often a multiphase fluid and contains gases such as H2S, CO2 or methane, which may diffuse through the pressure sheath. Gas diffusing through the sealing sheath of the flexible pipe increases the pressure in the annulus progressively with the diffusion. This pressure increase in the annulus may lead to problems of the internal sealing sheath collapsing, and this is so in particular in the case of a smooth-bore pipe in which said sheath is not supported by a carcass. This is, for example the case when the pressure in the annulus becomes very much greater than the pressure inside the pipe, as during a production shutdown or under certain particular conditions in service. Provision is also made for the gases present in the annulus to drain away in order to reduce the pressure therein. The gases drain away through and along the tensile armors toward a drainage valve generally located at a terminal end-fitting of the flexible pipe.
In the case of a smooth-bore pipe, the intermediate sheath located on top of the pressure vault prevents gas drainage into the tensile armor ply. Gas drainage therefore has to take place inside the pressure vault, but such a solution cannot be envisaged as it is very difficult for the gas in a layer such as the pressure vault, the winding angle of which is close to 90°, to be drained away effectively. Thus, smooth-bore flexible pipes are not used for transporting multiphase fluids or gases and are exclusively reserved for water injection lines, for which lines there is no gas diffusion problem. Thus, only rough-bore pipes are used for producing production lines, although these structures are more costly owing to the presence in particular of an additional expensive metal layer. Furthermore, the geometry of this layer is unfavorable for draining of the transported fluid.
Thus, there exists a real need for a low-cost structure of the smooth-bore type that can be used to transport gases or multiphase fluids. To try to satisfy this need, solutions have been proposed that consist in draining the gas closest to the internal pressure sheath. International Patent application WO 01/33130 describes a flexible pipe whose internal sheath has grooves in its external surface, said grooves being designed to drain away the gas between said sheath and the pressure vault. In the Assignee's patent application FR 01/11135 (not yet published), the internal pressure sheath is formed in two layers (double sheath) and the gas is drained away between the two sheaths in longitudinal grooves provided for this purpose. However, these solutions are very complicated to implement, especially because of problems due to creep of the thermoplastics used to produce the sheaths.
Another application (WO 99/66246), relating to bonded-type flexible pipes, mentions the flow of a liquid or gas above the pressure vault in an open or partly open space. However, there is no mention or even a suggestion of any solution to the problem of overpressure in the annulus due to gas diffusing through the pressure sheath.
In another patent (EP 0 937 932), the Assignee has disclosed a pipe with a structure that includes two tensile armor pairs and an intermediate sheath lying between the external armor pair and the internal armor pair, it being possible for such structure to be a smooth-bore structure. In such a structure the gas present in the internal annulus (the space lying between the internal sheath and the intermediate sheath) is drained away into the inner tensile armor pair, which has a lay angle of between 30 and 55°. However, this proposed solution does not make it possible to produce a low-cost smooth-bore pipe that can be used in production, especially because of the four tensile armor plies for the pipe, which increase its cost.
Moreover, whatever the type of pipe used (smooth-bore or rough-bore), the problem of gas diffusion means that the metal elements of the structure (pressure vault and tensile armors) that are located in the annulus must in particular be resistant to H2S. This entails a higher cost due to the particular treatments that they undergo, and the mechanical properties obtained remain modest.